Multilayer laminates are composed of various layers; in those layers, copper laminates alternate with organic insulating materials. The copper laminates, which contain the electrical circuit pattern, are black- or brown-oxidised at the metallic regions for better adhesion to the adjacent insulating layers. When the holes through which the metallic circuit patterns in different layers are connected to one another are being drilled in the assembled multilayers, slight damaging of the bond between the copper laminates and the adjacent insulating layers may occur in the region of those drill-holes because of mechanical or thermal effects. When the copper laminates are then subjected to an acid treatment to clean the walls of the drill-holes and in preparation for the subsequent metallisation, the acid can penetrate into the weakened joins between copper laminate and adjacent insulating layer and dissolve the copper oxide relatively rapidly. The acid is then able to advance further into the layer structure along the copper oxide layer; salt residues deposited there may result in short-circuits between the conductors. This phenomenon can be detected externally by the formation of rings around the drill-holes in the copper laminates, which rings appear pink and are therefore referred to among experts as "pink rings". The expression "pink rings" is understood as meaning a phenomenon in electrical multilayers that impairs their appearance and gives rise at least to doubts as to their ability to function and their reliability.
In order to avoid the formation of such pink rings and the adverse acid effects of which the pink rings are an indication, it is known to reduce the copper oxide on the metallic surfaces of the black- or brown-oxidised copper laminates by chemical reducing agents. The reduction generally takes place only partially, either to metallic copper or to monovalent copper, but with the desired adhesion-improving surface structure of the copper laminates being retained. The reduced surface layer cannot be attacked by the acid used in the subsequent process steps or, at all events, can no longer be attacked so rapidly.
The known reduction of the copper oxide on black- or brown-oxidised copper laminates which is effected by chemicals has a number of disadvantages. The reducing chemicals used are relatively expensive, and they become spent in the course of time. Furthermore, the chemical reduction takes a relatively long time: the reaction starts only with a certain delay after placing the copper laminates in the solution of the reducing agent. The long period of time required for the chemical reduction process is reflected in terms of the apparatus in long modules in which this process step is carried out. Both the costs of the chemicals used and the costs of the module are, therefore, relatively high. The running consumption costs required for the reduction are therefore significant.